The Bayer process, now more than 100 years old, extracts alumina from bauxite ore by contacting crushed or pulverized bauxite with a hot solution of caustic soda to dissolve the aluminum hydroxides contained therein as sodium aluminate. The remaining undigested insoluble residue, known as red mud, is separated from the solution usually by filtration or sedimentation or both.
Red mud typically includes large quantities of finely divided solids resistant to separation. Alumina manufacturing therefore creates a need for improved separation systems, particularly those that can rapidly separate large quantities of slurries in an efficient manner.
In this connection, some preliminary attempts appear to have been made to use pressurized settling devices in clarifying finely divided solids from slurries. U.S. Pat. No. 2,107,919 (Turner) appears to discuss a pressure digester placed between two blow off tanks. U.S. Pat. No. 4,994,244 (Fulford) states that separation of red mud from the digested slurry can be carried out at a temperature below the boiling atmospheric temperature of the liquor phase of the slurry. U.S. Pat. No. 5,080,803 (Bagatto) discusses a process and apparatus for decanting suspensions at and above the atmospheric boiling point of the suspension. The apparatus operates at atmospheric pressure.
While the foregoing references discuss attempts to increase separation efficiency of finely divided solids from slurries, there exists a need, particularly in the aluminum industry, for improved systems to carry out separation at high pressure and temperature.